Optical inspection can be used to determine properties of particulate material, for example colour and reflectivity. One particular application of optical quality measurement is in monitoring the cosmetic quality of synthetic diamond crystals that are to be used as abrasive material. In one known measurement system, a sample is placed in a pot, that is to say an open container, and the exposed surface of the material is inspected by a suitable optical measurement device. Usually, the sample surface is uneven, and its height relative to the measurement device is inconstant, depending on the size of the particles. In the case of saw diamond abrasive, even though each sample is taken from a specific size range, there is still a variation of size and shape of particles within the sample. The resulting uneven sample surface and variations in surface height relative to the measurement device materially reduce the accuracy and repeatability of the measurement results, and measurement on one sample may not be readily comparable with measurements on other samples.